Bolted joints are used to affix two or more parts together and typically have two engaging elements (a bolt and nut or equivalent structure) that are characterized by cooperative threads. The threads are most often spiral and have a given pitch and thread count. Relative longitudinal movement between the nut and bolt along a common axis is produced when one or the other element is rotated. At some longitudinal position, the two elements engage the parts that are to be fastened, and fix the same in place when sufficient torque is applied to one element relative to the other. The parts remain fixed due to the frictional force that is generated at the interfaces between threads, the parts and the surfaces of the engaging elements (nut and bolt) that contact the parts. This type of fastened joint suffers from the problem of unintentional loosening due to operational conditions. Tightened bolts or nuts rotate loose when relative motion between the cooperating threads occurs.
Several factors can cause this relative motion. First, parts can bend which results in forces being induced at the friction surface. Second, changes in temperature can cause the bolt and nut to alternatively constrict and expand, causing slight relative movement that reduces those frictional forces. Finally, applied forces on the joint components can lead to shifting of the joint surfaces.
Many attempts have been made to prevent a bolt from being loosened unintentionally. Some attempts involve the increase of friction between the male and female threads. Other attempts try to increase the frictional force between the bottom of the bolt head and the work piece by the use of extra components such as washers interposed between an engaging element and the part that it contacts. These can be split, star or spring washers. Others have added a chemical adhesive to the threads to bond the nut to the bolt at the threads.
However, these attempts have several disadvantages. First, these attempts still suffer from relative loosening when different forces are applied. Second, an increase in friction often takes more torque to install than a standard nut and bolt. Third, the extra parts or chemicals significantly increase assembly cost and time. The present invention overcomes these disadvantages by providing a bolt that is as easy to install as a standard assembly but does not suffer from loosening over time.